Wireless communication systems are widely used for many different purposes. More and more people every day purchase cellular telephones or other wireless communication devices, including but not limited to pagers, computers, and Personal Digital Assistants (PDA's). These devices can be capable of receiving and transmitting information using a communication system such as a cellular network.
One place people use their communication devices is when traveling (for example, when driving in a car, riding a bus, or riding in a taxi). In operation, the communication device typically receives a signal from a first cell of the cellular system. Next, the communication device will receive signals from the first cell and additionally a second cell as the communication device moves toward the second cell and away from the first cell. Finally, as the communication device continues to move away from the first cell and toward the second cell the communication device will only receive a signal from the second cell. This scenario is what is known in Code Division Multiple Access (CDMA) systems as a soft handoff or handover (that is, a smooth, planned, and transparent switch from one cell to another). Though such soft handoffs are desirable, it is estimated that a soft handoff occurs only 40% of the time when a communication device is being used inside a moving vehicle.
In normal conditions, when moving from a first cell toward a second cell, the signal from the first cell becomes weaker, while the signal from the second cell becomes stronger. When the signal from the second cell is stronger than a threshold Tadd, the communication device starts communicating with the base station in the second cell while also communicating with the base station in the first cell. The communication device will communicate with both cells until the signal from the first cell drops bellow a threshold Tdrop. During this transition period, the mobile is vulnerable, and adverse RF conditions may cause, for example, the call to drop.
A problem can occur when the mobile moves from a first cell to a second cell for the scenario shown in FIG. 1 (further description regarding FIG. 1 is provided below). In this scenario, the signal from the second cell is partially blocked by a building. As a car carrying a wireless mobile station travels toward the second cell, the signal from the second cell will increase enough to exceed the Tadd threshold causing the mobile to enter a soft handoff. Nevertheless, while the signal from the first cell becomes weaker, the signal from the second cell remains low, due to a large building that blocks the signal from the second cell. In this situation the mobile's signal to interference (S/I) experience will decrease and a request for more power from the base stations will be made by the communication device via a fast forward power control loop as is well understood in the art. Since both base stations within these cells will receive the same command, they will both increase their power by the same ratio, and, since the signal from the first cell was a-priori stronger, power drift will typically take place. Nevertheless, the increase in power will not be very helpful, since the communication device is moving away from the first cell and is still in the shadow of the building for the signal from the second cell. As a result, the communication device will send another request for power increase, causing further power drift. This can continue to happen as long as the communication device is in the shadow of the building. As a result, two things may happen. First, unless the communication device moves from the shadow of the building, the handoff may fail and the call may drop. Second, the increase in the power of the first cell, while not very helpful for the mobile, may cause interference to other cells. Both of these possibilities are undesirable.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions, sizing, and/or relative placement of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.